diff --git a/pandas/core/generic.py b/pandas/core/generic.py
index c20b2840a40ab..2e34bc0563210 100644
--- a/pandas/core/generic.py
+++ b/pandas/core/generic.py
@@ -11143,7 +11143,7 @@ def __ixor__(self, other):
     # Misc methods
 
     @final
-    def _find_valid_index(self, how: str):
+    def _find_valid_index(self, *, how: str) -> Optional[Hashable]:
         """
         Retrieves the index of the first valid value.
 
@@ -11156,16 +11156,16 @@ def _find_valid_index(self, how: str):
         -------
         idx_first_valid : type of index
         """
-        idxpos = find_valid_index(self._values, how)
+        idxpos = find_valid_index(self._values, how=how)
         if idxpos is None:
             return None
         return self.index[idxpos]
 
     @final
     @doc(position="first", klass=_shared_doc_kwargs["klass"])
-    def first_valid_index(self):
+    def first_valid_index(self) -> Optional[Hashable]:
         """
-        Return index for {position} non-NA/null value.
+        Return index for {position} non-NA value or None, if no NA value is found.
 
         Returns
         -------
@@ -11176,12 +11176,12 @@ def first_valid_index(self):
         If all elements are non-NA/null, returns None.
         Also returns None for empty {klass}.
         """
-        return self._find_valid_index("first")
+        return self._find_valid_index(how="first")
 
     @final
     @doc(first_valid_index, position="last", klass=_shared_doc_kwargs["klass"])
-    def last_valid_index(self):
-        return self._find_valid_index("last")
+    def last_valid_index(self) -> Optional[Hashable]:
+        return self._find_valid_index(how="last")
 
 
 def _doc_params(cls):
diff --git a/pandas/core/missing.py b/pandas/core/missing.py
index c2193056cc974..41d7fed66469d 100644
--- a/pandas/core/missing.py
+++ b/pandas/core/missing.py
@@ -158,7 +158,7 @@ def clean_interp_method(method: str, **kwargs) -> str:
     return method
 
 
-def find_valid_index(values, how: str):
+def find_valid_index(values, *, how: str) -> Optional[int]:
     """
     Retrieves the index of the first valid value.
 
@@ -256,8 +256,17 @@ def interpolate_1d(
 
     # These are sets of index pointers to invalid values... i.e. {0, 1, etc...
     all_nans = set(np.flatnonzero(invalid))
-    start_nans = set(range(find_valid_index(yvalues, "first")))
-    end_nans = set(range(1 + find_valid_index(yvalues, "last"), len(valid)))
+
+    first_valid_index = find_valid_index(yvalues, how="first")
+    if first_valid_index is None:  # no nan found in start
+        first_valid_index = 0
+    start_nans = set(range(first_valid_index))
+
+    last_valid_index = find_valid_index(yvalues, how="last")
+    if last_valid_index is None:  # no nan found in end
+        last_valid_index = len(yvalues)
+    end_nans = set(range(1 + last_valid_index, len(valid)))
+
     mid_nans = all_nans - start_nans - end_nans
 
     # Like the sets above, preserve_nans contains indices of invalid values,
@@ -595,8 +604,12 @@ def _interpolate_with_limit_area(
     invalid = isna(values)
 
     if not invalid.all():
-        first = find_valid_index(values, "first")
-        last = find_valid_index(values, "last")
+        first = find_valid_index(values, how="first")
+        if first is None:
+            first = 0
+        last = find_valid_index(values, how="last")
+        if last is None:
+            last = len(values)
 
         values = interpolate_2d(
             values,